Because of climate change, flood-prone areas are more and more frequently exposed to potential casualties and damage. The capability of the flow to carry relevant quantities of sediments during these critical events adds to the further complexity of the resulting scenarios. The interaction between the flow and the obstacles in flood-inundated areas contributes to an increase in the hazard level and constitutes a relevant concern in the framework of risk analysis. Despite this relevance, the existing literature on the topic is relatively scarce, especially for the estimation of the forces acting on rigid obstacles in the presence of a mobile bed. In the present paper, a recent two-phase shallow-water morphodynamical model, particularly suited for the analysis of fast geomorphic transients, is applied for the numerical simulation of the propagation of a dam-break wave over an erodible floodplain in the presence of a rigid obstacle. The geometry of the test-case is inspired by a recent fixed-bed study reported in the literature, for which extensive experimental and numerical data concerning the flow field and the dynamic loading against the obstacle are available. Results of the numerical simulations contribute to highlight the effects of the obstacle on the changes in the bottom topography.